Scrambling Techniques

Although the bi$-$phase techniques have achieved widespread use in local area network applications at relatively high data rates $($up to $10$ Mbps$),$ they have not been widely used in long$-$distance applications. The principal reason for this is that they require a high signaling rate relative to the data rate. This sort of inefficiency is more costly in a long$-$distance application.

Another approach is to make use of some sort of scrambling scheme. The idea behind this approach is simple: Sequences that would result in a constant voltage level on the line are replaced by filling sequences that will provide sufficient transitions for the receiver$$s clock to maintain synchronization. The filling sequence must be recognized by the receiver and replaced with the original data sequence. The filling sequence is the same length as the original sequence, so there is no data rate penalty. The design goals for this approach can be summarized as follows:

$$ No dc component

$$ No long sequences of zero$-$level line signals

$$ No reduction in data rate

$$ Error$-$detection capability

Two techniques are commonly used in long$-$distance transmission services; these are illustrated in the Figure above.

A coding scheme that is commonly used in Europe and Japan is known as the high$-$density bipolar$-3$ zeros $($HDB$3)$ code $($Table below$).$ As before, it is based on the use of AMI encoding. In this case, the scheme replaces strings of four zeros with sequences containing one or two pulses. In each case, the fourth zero is replaced with a code violation.